Conventional read-only memory (ROM) circuits are implemented as special-purpose integrated circuits for the permanent storage of program instructions and electronic data. For example, a ROM circuit can be manufactured with specific instructions for the operation of a computer system.
Typically, a ROM circuit consists of an array of memory cells on a semiconductor, and each memory cell has a transistor that is fabricated to indicate a logic “one” or a logic “zero” based on how the semiconductor is implanted to create the transistor. The data is permanently stored with a memory cell, and it cannot then be erased or altered electrically. Each of the transistors can be formed so as to have one of the two predetermined logic values.
A programmable ROM (PROM) circuit is designed with memory cells having programmable memory components that can be programmed after the semiconductor chip has been manufactured. The memory cells of a PROM device are programmed with data (e.g., a logic one or a logic zero) when the data is burned into the chip. This is accomplished by forming contacts that define the threshold voltage levels near the end of the manufacturing process, or after the manufacturing process. When a PROM device is programmed, the device can be implemented like a conventional ROM chip in that the data cannot be electrically altered.
Due to the costs of fabricating semiconductor memory devices, and the design of smaller integrated circuit-based electronic devices, there is an ever-present need to provide non-volatile memory circuits that take up less space, have improved memory storage capacity, and are inexpensive to manufacture.